1. Technical Field of the Invention
The present invention relates to starters for starting up internal combustion engines mounted on vehicles and, more particularly, to a starter having an overrunning clutch.
2. Related Art
In normal practice, a starter starts up an internal combustion engine of a vehicle when supplied with electric power from an on-vehicle battery to be rotated. As disclosed in Japanese Patent Laid-open (unexamined) Patent Publication No. 7-37786, one type of such a starter includes one that has an overrunning clutch. Such a starter includes a motor (starter motor), a drive shaft having an outer periphery formed with helical shaft splines through which a drive power is transmitted from the starter motor, an overrunning clutch with an inner periphery formed with helical bore splines in mesh with the helical shaft splines and moveable in an axial direction with respect to the drive shaft while serving as one-way clutch, and a pinion gear integrally disposed on the overrunning clutch and operative to engage with and disengage from a ring gear of the internal combustion engine. Thus, the drive shaft, through which the drive power is transmitted from the starter motor, and the overrunning clutch are held in spline coupling.
By the way, looseness (clearance) is present between both splines in a radial and circumferential direction. Therefore, a probability occurs for the overrunning clutch to incline with respect to the drive shaft. In particular, under circumstances where an ignition switch is turned on once while, subsequently, the ignition switch is turned off, before the internal combustion starts up, and turned on again, inertial rotation of the starter motor causes the pinion gear to mesh with the ring gear of the internal combustion engine with the pinion gear remaining under rotation. This results in rapid stop of rotation of the pinion gear. In this case, spline-coupling portions between the drive shaft and the overrunning clutch encounter undue impacts to cause the overrunning clutch and the drive shaft to incline with respect to each other.
As a result of the occurrence of such inclinations of the overrunning clutch and the drive shaft, the spline-coupling portions have surfaces in local contact. Accordingly, this results in undue surface pressures with a fear of the occurrence of adhesion between contact areas of both splines.